first off, with a tank that size, if you dont cycle, you can do 100% water changes maybe every 8 days, if you cycle you could put it down to 30% each week. (you'll need a good filter though)
i would try cycle if i were you :) saves you work.

also, you'll need a heater(adjustable)... bettas NEED heaters unless you live in a really warm country... your water should always be around 75 - 77 degrees F. best get a thermometer you stick on the glass too of course.

^^^ agree with Jupiter.
AquaOne is a really good brand and imo its not expensive either. but really you can get whatever brand you can caus i think there are no bad heaters out there anymore really... they are all fairly decent now-a-days.

your water is probably cloudy from the gravel....
dont worry.

have you used water conditioner??? its VERY important you get some water treatment. i recommend "Interpet fresh start".

so on your shoppinglist;

Water conditioner (if you have none. take fish out, add it accordingly to instructions, mix, and put fish back in)Heater, adjustable
Stick on thermometer

I wouldn't recommend a stick on thermometer. They tell you the temperature of the glass of the tank and not the water. I would recommend one that you place in the tank with a suction cup or the weighted ones that sit on the bottom.

I think it's a very nice tank. :) Your boys will be very happy. Keep an eye on that divider to make sure neither can get at each other. You would be surprised how high they can jump and such smalls gaps you can fit through. Also keep an eye out that those plastic plants don't catch their fins and tear them up.

Any adjustable heater will work fine. Just don't buy the one that is preset to a certain temperature. I have several Hydor Theo's and I think they work great and are fairly priced.

I use Prime for water conditioner as it's more condensed and saves me money in the long run. There are plenty out on the market though!

i have water conditioner and there is no way for those fish to get at each other. I made sure of that lol.
as for the plants, only one of them is plastic, the others are silk plants. I was actually talking to my friend who works at a pet store, and she said that I dont need a heater, so there is a lot of confusion as to what i "really need".
So i got a sticker thermometer and i will see what room temp. in my hosue is :)

the water is still cloudy but clearing up!

BTW we bought another fish lol. My boyfriend will be joining here ASAP!

Betta fish are cold blooded animals as well as being tropical species. They need warmth to aid in metabolism. I would listen to experienced betta keepers who are only trying to give you the best advice available.
It's the same concept of giving your reptile a heat source. So it can warm up and digest its food.
I've gotten a lot of poor advice from pet store employees, even resulting in a death of one of my reptiles. :(

Get a heater. Pet store employees have a quota (at least my local pet store does) of how many animals they must sell in a month or they get fired. So of course they'll tell you some stupid things to make the animal seem easier to care for to make a sell.

My betta actually acts sick and sluggish when the water gets below 75 degrees. He literally will stop swimming and just sit there.